The present invention relates to a series motor with commutator, in particular to a universal motor for a braked power tool, having switching means for switching between motor mode and braking mode, where, in motor mode, at least one field winding is connected in series with an armature winding in a motor circuit to which a supply voltage is supplied, and where, in braking mode, the at least one field winding and the armature winding form together a closed braking circuit disconnected from the supply voltage.
A series motor of this kind is known from EP 0 471 038 B1.
The known motor is particularly well suited for driving braked power tools, such as braked right-angle grinders or circular saws, for which purpose it uses a multipole changeover switch by means of which the motor is short-circuited, and the field winding is polarity-reversed, in braking mode, and further means for limiting the braking current by the field winding. This is to permit smooth and rapid short-circuit braking by autonomous self-excitation. The armature is connected in motor mode between the field windings and the commutating windings, while in braking mode a current path is established between the armature and the commutating windings that comprises a Zener diode arrangement for limiting the braking current so that only a certain portion of the braking current is allowed to flow through the field windings.
Although such a motor permits relatively reliable braking of the motor when the latter is switched over to the braking mode, it has been found that in certain individual cases the braking action nevertheless becomes active only with reducing speeds. The reason for this is seen in the fact that in the case of motors that do not commutate perfectly a relatively high transition resistance may exist between collector and brushes, in particular at high speeds and in cases of a slightly out-of-round commutator. In such cases, the remanence is not sufficient for the autonomous self-excitation to produce a braking effect.
One has already tried to circumvent such problems by using a capacitor which is charged, in motor mode, and utilized in braking mode to initiate autonomous self-excitation (DE 36 36 555 A1).
A disadvantage of this arrangement lies, however, in the electronics which are inherently prone to failure, which has the result to reduce the functional safety of the brake. In addition, the capacitor charge can be used only once to initiate the braking action. If the capacitor charge does not suffice to initiate the braking action, than initiation of a braking action can be expected all the less after discharge of the capacitor.
U.S. Pat. No. 2,818,542 describes a shunt-wound d.c. motor intended for stationary use. For amplifying the braking effect in case of activation of the brake, a rectified a.c. voltage, generated via a generator or a transformer, is superimposed in this case on the operating voltage in order to produce an increased magnetic flux for the purpose of achieving rapid braking when the braking action is activated.
A motor of this kind is in no way suited to serve as drive for a braked power tool. Further, such an application requires that a series motor with commutator, preferably a universal motor, be used and that excessively abrupt braking be avoided, as this would result in a considerable risk potential, especially in the case of right-angle grinders.
It is a first object of the present invention to provide an improved series motor which improves the reliability of the braking action when changing over from motor mode to braking mode.
It is a second object of the invention to provide an improved series motor having less brush sparking.
It is a third object of the invention to provide an improved motor being less prone to failure when braking.
It is a fourth object of the invention to provide an improved series motor with a reliable braking mode which is simple and easy to manufacture.
These and other objects are achieved in a series motor of the type described at the outset by a mains-operated transformer whose secondary winding is connected within the braking circuit.
The secondary winding of the transformer has the effect to introduce an a.c. current into the braking circuit which is sufficient to initiate reliable self-excitation of the motor. The remanence d.c. voltage induced in the armature winding and the a.c. voltage originating from the transformer current are superimposed one on the other.
The invention ensures in this way rapid initiation of the braking action when changing over to braking mode, even under extremely unfavorable conditions in the presence of high transition resistance values between collector and brushes.
According to an advantageous further development of the invention the at least one field winding, in braking mode, forms a closed circuit with at least one commutating winding and with the armature winding.
This feature leads to a further improvement of the braking action of the motor. The commutating winding may be given a relatively small cross-section, as current flows through it only in braking mode, not in motor mode.
According to a further embodiment of the invention the secondary winding of the transformer, in braking mode, is connected in series, in the braking circuit, to the at least one field winding.
In this embodiment, the secondary winding of the transformer is passed by the braking current in the braking mode so that it must be appropriately sized, i.e. in a usual universal motor operated at 230 Volts for a current flow in the order of 5 Amperes. The transformer for such a universal motor is so designed that the secondary voltage is preferably of the order of approximately 2 to 3 Volts, preferably approximately 2.5 Volts.
According to another embodiment of the invention, the secondary winding of the transformer, in braking mode, lies in the braking circuit in a circuit parallel to the field winding.
Preferably, the secondary winding is in this case connected in series to a resistor so as to limit the current flow through the secondary winding of the transformer.
This allows a commercially available transformer to be used since the secondary winding of the transformer is not directly passed by the braking current. When the collector/brush resistance is high, then the secondary current flows through the field winding which has the result to increase the induced voltage in the armature. The higher induced voltage in the armature has the effect to safely initiate the braking action. As the deceleration of the motor progresses, the transition resistance between collector and brushes clearly drops so that at a later point in time the main current flow occurs through the armature winding, whereas the current flow through the secondary side of the transformer can be limited by the resistance.
All in all, the secondary winding of the transformer can be designed in such a way that the effective secondary voltage is in the range of approximately 3 Volts, for example, in which case a series resistor of 20 Ohms is used to achieve an effective current flow of approximately 150 mA. A commercially available transformer of small size will be sufficient for this purpose.
According to an advantageous further development of the invention, limiting means are provided in the braking circuit for limiting the voltage induced at the armature winding.
This feature has the effect to reduce brush sparking.
The limiting means can be designed to limit the voltage seen by the at least one field winding so that the latter is limited to approximately 5 Volts, for example.
The secondary winding of the transformer may be connected in this case with one of its ends to the at least one field winding and with its other end to the armature winding and one end of the limiting means.
Alternatively, the armature winding and the at least one commutating winding are connected in series in the braking circuit and are connected to the at least one field winding, in parallel to the limiting means and the current path formed through the secondary winding of the transformer.
According to a first embodiment, the limiting means is configured as diode path with reverse-parallel connection of the diodes.
According to a further embodiment of the invention, a diode is connected in series with the at least one field winding in the braking circuit.
In this way, the direction of current in the braking circuit is impressed by the diode during self-excitation, irrespective of the existing remanence.
This provides the advantage that the limiting means may be designed as simple diode path, which results in cost savings.
According to a further embodiment of the invention, a diode is connected in series to the secondary winding of the transformer.
This feature has the result that only one half-wave of the a.c. voltage is used for the transformer, i.e. that the transformer is operated with only half the effective voltage in the secondary circuit, which leads to a reduction of the power loss, compared with the use of a resistor in the secondary circuit. It is thus possible to use a low-cost, commercially available transformer.
According to a further variant of the invention, the limiting means comprises a transistor circuit which produces a current flow parallel to the at least one field winding when a threshold voltage is reached at the at least one field winding.
A circuit if this type can be realized at clearly lesser cost than a correspondingly sized diode path.
A particularly advantageous embodiment is obtained when the limiting means comprises a field effect transistor that switches through when a threshold voltage is reached in order to limit the voltage dropping at the at least one field winding.
If the motor is designed for a rated operating voltage of 230 Volts a.c. and a motor output of approximately 2.500 Watts, it is possible with advantage to make direct use of a particular property of a commercially available MOSFET, namely that it will become conductive only when a threshold voltage of between approximately 4.5 and 5 Volts is reached between drain and source. In this case, the MOSFET is connected in the braking circuit, with gate and drain connected to one end of the at least one field winding and with source connected to the other end of the at least one field winding, there being of course provided in the braking circuit a diode for impressing the direction of current. If the voltage-limiting action procured by the MOSFET is to start already at a lower threshold value (logic level MOSFET) of, for example, below 4.5 to 5 Volts, then a special MOSFET with a lower threshold voltage can be used that becomes conductive at a lower threshold voltage of approximately 2 Volts. This reduces the intensity of the braking action.
It is understood that a voltage divider may of course also be used for driving the MOSFET if the voltage-limiting action is to start only when a higher threshold voltage is reached. In this case, the braking characteristics can be adjusted by appropriately adapting the voltage divider to the desired value.
According to a further variant of the invention, a bipolar transistor is used for the limiting means, which may have its base connected to the voltage divider. Since the threshold value of a bipolar transistor is in the range of approximately 0.7 Volts, it is necessary in this case to use a voltage divider which must be suitably designed to produce the desired braking characteristics. There is also a possibility to drive the bipolar transistor via a Zener diode at its base.
It is understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the context of the present invention.